Two nonhuman primate efficacy studies have convincingly demonstrated that CMV/SIV vectors can: 1) reinfect CMV+ rhesus macaques (RM), 2) during re-infection, elicit potent and persistent SIV-specific CD4+ and CD8+ T cell responses with a strong effector memory (T{EM}) bias, and 3) completely protect ~50% of vaccinated RM from progressive SIV infection after limiting dose rectal challenge with the highly pathogenic SIVmac239 virus. The protection manifested in these RM is distinct from previous vaccines in its abruptness and extent, with protected RM exhibiting a viral burst in plasma of varying size upon initial infection, followed by immediate control to undetectable levels. Protection correlates with the extent of total SIV-specific CD8+ T cells generated during the vaccine phase, and is stable in the vast majority of protected RM for >12 months. These data indicate a novel pattern of protection consistent with very early control, likely taking place at the site of viral entry and/or early sites of viral replication and amplification, and involving tissue-resident CD8+ T{EM}- Thus, CMV vectors and the T{EM} vaccine concept offer a powerful new approach to HIV/AIDS vaccine development, and have the potential to be developed into a safe and effective HIV/AIDS vaccine. In this Program, we seek to: 1) increase the potency of CMV/SIV vectors so as to achieve rates of protection closer to 100% of vaccines, 2) reduce the pathogenic and shedding potential of CMV vectors, while retaining immunogenicity, so as to achieve an effective vaccine that is safe enough for use in a general human population, and 3) determine immunologic correlates or protection to guide further development of the T{EM} vaccine concept. The program is composed of 3 projects, and 5 cores. Projects 1 and 2 will use novel strategies to develop replication-deficient and tropism-modified CMV vectors that will retain immunogenicity, but have reduced shedding and capacity to mediate disease. Project 3 seeks to enhance CMV/SIV vector immunogenicity with both combinatorial vaccine approaches and CMV vector modification, and will determine immunologic correlates ofthe novel all or none protection associated with these vectors. These projects will be assisted by Core A (Administration), Core B (Nonhuman Primate), Core C (Pathogenesis Models), Core D (Virology and Immunology Monitoring), and Core E (Genomics).